Environmental and occupational exposure to lead place large numbers of children and adults at risk of lead poisoning. Because the central nervous system toxicity of lead may be irreversible, its is desirable to screen populations at risk for increased lead absorption in order to detect otherwise asymptomatic poisoning. The determination of elevated levels of zinc protoporphyrin (ZPP) in erythrocytes has been used for some years as a primary screening procedure for lead exposure. ZPP is also elevated, and useful in diagnosis, in iron deficiency. The logistics of screening would be simplified if a noninvasive technique could be developed which would measure erythrocyte ZPP in vivo without the need for a blood sample. An electro-optical device designed for this purpose will be tested; it uses green light for transcutaneous excitation of ZPP fluorescence, and determines the volume of subcutaneous blood from green, yellow, and red reflectance measurements, deriving a blood ZPP level from the ratio of measurements of ZPP fluorescence and blood volume. Flow cytometry will be used to investigate distributions of ZPP fluorescence in individual erythrocytes.